The present invention relates to a resin boot used for constant velocity universal joints.
The majority of resin boots for constant velocity universal joints were made of a rubber material such as chloroprene rubber (CR), but in recent years resin boots have often been employed because of their reliability in durability, heat resistance, cold resistance, high speed rotatability, and resistance to percussion of foreign matter.
FIGS. 3a and 3b show an example of a conventional resin boot. This resin boot is fixed at its larger diameter portion 11 to the outer race 21 of a constant velocity universal joint by a boot band 22 and at its smaller diameter portion 12 to a shaft 23 by another boot band 22. A bellows portion 13 is interposed between the larger and smaller diameter portions 11 and 12 and comprises seven ridges 13a (13a1, 13a2 , . . . 13a7 as seen from the smaller diameter portion 12), six troughs 13b (13b1, 13b2, . . . 13b6 as seen from the smaller diameter portion 12) and slopes 13c which connect the ridges 13a and troughs 13b.
Resin boots are harder (generally, about H.sub.D 50) than rubber boots. Therefore, for a resin boot having the same bellows portion length as that of a rubber boot, if the joint assumes a large operating angle, high tensile strains (strains due to tensile stress) occur in the troughs 13b on the stretch side. Thus, it has been common practice to make the length of the bellows much longer than that of a rubber boot, to use larger numbers of ridges and troughs (seven ridges and six troughs) and fix the resin boot to the joint in a state in which it is somewhat compressed from its free length L'1 (percentage compression; (L'1-L'2)/L'1 is 23-26% or so, where L'2 is the as-installed length), so as to avoid large tensile strains occurring in the troughs 13b even if the joint assumes a large operating angle. As a result, the resin boot has large radial and axial dimensions as compared with the rubber boot.
Further, the conventional resin boot is shaped such that in relation to the thickness ratio between the ridges 13a and troughs 13b (trough/ridge=1.5-2.1 or so) and to the balance of thickness between the ridges and troughs (see Table 1), most of the compression load at the time of mounting is absorbed by the first trough 13b1 while the remaining troughs 13b2 through 13b6 are not compressed so much, the amount of compression decreasing as the larger diameter side is approached (see Table 2).
TABLE 1 ______________________________________ the thickness of the ridges (mm) (the peripheral minimum value-maximum value) 1st 2nd 3rd 4th 5th 6th 7th ______________________________________ 0.80.about. 0.55.about. 0.60.about. 0.75.about. 0.75.about. 0.75.about. 1.30.about. 1.75 0.95 1.00 1.15 1.15 1.15 2.10 ______________________________________ the thickness of the troughs (mm) (the peripheral minimum value-maximum value) 1st 2nd 3rd 4th 5th 6th ______________________________________ 1.15.about. 1.25.about. 1.40.about. 1.35.about. 1.40.about. 1.30.about. 1.60 1.65 1.80 1.75 1.80 2.00 ______________________________________
TABLE 2 ______________________________________ the percentage compression between the ridges (%) {(L1 - L2)/L1} 1st-2nd 2nd-3rd 3rd-4th 4th-5th 5th-6th 6th-7th ______________________________________ 71 40 27 23 24 20 ______________________________________
Table 1 shows the levels of the thickness of the ridges and troughs of a bellows portion in a conventional resin boot.
Table 2 shows the percentage compression between ridges during attachment of the conventional resin boot. For this reason, as the boot is axially compressed, the slope 13c extending from the first ridge 13a1 to the first trough 13b1 (said slope being designated by 13c1) and the slope 13c extending from the first trough 13b1 to the second ridge 13a2 (said slope being designated by 13c2) approach each other, resulting in the slope 13c2 being rapidly tilted toward the larger diameter portion 11 rather than becoming perpendicular to the center line X of the boot. This phenomenon, when seen in a compressive load versus amount of axial compression graph shown in FIG. 8, appears as a remarkable point of inflection; the compression load sharply decreases during the compression (loading) process and sharply increases during the load removing process. The durability of the boot is highest in the vicinity of the point of inflection and decreases as the compression proceeds from the point of inflection. The reason is that when the joint assumes a large operating angle, the first trough 13b1 in the stretch side which has a greater amount of compression when attached does not stretch so much because of the presence of the point of inflection (because the compression load sharply increases at the point of inflection) and hence the correspondingly greater tensile load acts in the remaining second trough 13b2 through sixth trough 13b6, producing higher tensile stresses. Particularly, the fifth trough 13b5 and sixth 13b6 having less compression produced therein when attached have greater bending stresses (difference between tensile stress produced in the stretch side and compression stress produced in the compression side) and sometimes the boot life depends largely on the bending fatigue of the fifth and sixth troughs.
As described above, the durability of the resin boot depends largely on the bending fatigue of the bellows portion and this problem has heretofore been coped with by increasing the numbers of ridges and troughs of the bellows portion. Therefore, the shape of the resin boot has necessarily been far larger than that of the rubber boot. Reversely, to make the shape compact, it is necessary to decrease the numbers of ridges and troughs; however, if this measure is actually taken with the conventional design concept retained, greater bending stresses will be produced in the troughs closer to the larger diameter portion, leading to a decrease in durability. That is, it should be said that in the prior art, increased durability of the boot is in inverse relation to its compact shape.